<i18n dev> [15] RFR: 8244459: Optimize the hash map size in LocaleProviderAdapters
Peter Levart
peter.levart at gmail.com
Tue May 5 19:12:37 UTC 2020
Hi Martin,
On 5/5/20 8:26 PM, Martin Buchholz wrote:
> See related:
> https://guava.dev/releases/23.0/api/docs/com/google/common/collect/Maps.html#newHashMapWithExpectedSize-int-
This is basically the same calculation (or at least gives same result)
as Naoto did (without the max part):
Naoto: (int)(expectedSize / 0.75f) + 1
Guava: (int) ((float) expectedSize / 0.75F + 1.0F)
but in case expectedSize is a multiple of 3, it gives the result which
is 1 more than needed. If what is needed is also a power of 2, then
twice the needed space is allocated in the HashMap backing table.
Regards, Peter
>
> On Tue, May 5, 2020 at 11:03 AM <naoto.sato at oracle.com
> <mailto:naoto.sato at oracle.com>> wrote:
>
> And here is the fix. Please review.
>
> http://cr.openjdk.java.net/~naoto/8244459/webrev.00/
>
> Naoto
>
> On 5/5/20 10:25 AM, naoto.sato at oracle.com
> <mailto:naoto.sato at oracle.com> wrote:
> > Hi Peter,
> >
> > You are correct. Thanks. I'll remove that initial value of 16.
> >
> > Naoto
> >
> > On 5/5/20 9:37 AM, Peter Levart wrote:
> >> Hi Naoto,
> >>
> >> On 4/30/20 12:18 AM, naoto.sato at oracle.com
> <mailto:naoto.sato at oracle.com> wrote:
> >>> Hello,
> >>>
> >>> Please review this small fix to the following issue:
> >>>
> >>> https://bugs.openjdk.java.net/browse/JDK-8244152
> >>>
> >>> The proposed changeset is located at:
> >>>
> >>> https://cr.openjdk.java.net/~naoto/8244152/webrev.00/
> >>>
> >>> The hash map used there didn't have initial capacity, even
> though the
> >>> exact numbers are known.
> >>
> >>
> >> Well, it has to be calculated 1st (countTokens), but I guess
> this pays
> >> off when HashSet (the backing HashMap) does not have to be
> rehashed then.
> >>
> >> The expression you use:
> >>
> >> Math.max((int)(tokens.countTokens() / 0.75f) + 1, 16)
> >>
> >> ...has a minimum value of 16. Why is that? 16 is just HashMap's
> >> default initialCapacity if not specified explicitly. But if you
> only
> >> want to store say 1 entry in the map, you can specify 2 as
> >> initialCapacity and HashMap will happily work for such case
> without
> >> resizing.
> >>
> >>
> >> So you could just use:
> >>
> >> (int)(tokens.countTokens() / 0.75f) + 1
> >>
> >> And even this expression is sometimes overshooting the minimal
> >> required value by 1 (when # of tokens is "exact" multiple of
> 0.75f,
> >> say 6). I think the following could be used to optimally
> pre-size the
> >> HashMap with default load factor 0.75:
> >>
> >> (tokens.countTokens() * 4 + 2) / 3
> >>
> >>
> >> Regards, Peter
> >>
> >>>
> >>> Naoto
>
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